The solid-state look-around (SSLA) endoscope incorporates very wide-angle optics which capture a large field of view. A SSLA endoscope typically has an actual angle of view which is non-zero (meaning the scope lens is tilted relative to the axis of the scope rod). During a procedure, a doctor will select a different display angle of view from a range of programmed possibilities. An unwarping and rotation algorithm may then simulate an image as if the actual viewing angle were the same as the selected display angle of view. In such simulation, a large portion of the captured image (also called the sensor image) is not used, but rather only the pixels inside a region of interest (ROI) are used to form the displayed image. The captured image typically includes warping or distortion due to the use of a wide angle lens. Such distortion may include highly non-linear barrel distortion in which the image magnification increases non-linearly with the distance from the axis of the optical system. The distortion typically needs to be corrected to view useful images from the endoscope. The correction requires processing the image using knowledge of the distortion resulting from the wide angle lens and other optical elements, typically with a relatively slow image processing technique that non-linearly resamples the image to produce the unwarped displayed image data.
To obtain quality images using endoscopes, exoscopes, borescopes, or other imaging devices, the image exposure must be controlled. This involves adjusting elements such as the light source, light source intensity, the optical assembly aperture, and the shutter speed or time for which image sensor integrates charge for each image frame. Most prior art methods for exposure control include steps wherein the number of image pixels that have a digital value above or below predetermined thresholds are counted. If the number is too high, the exposure is controlled downward (faster exposure) to avoid overexposure, and if the number is too low, the exposure in controlled upward to avoid underexposure. When device produces distorted images, it is the undistorted version that should best be used to produce accurate counts of the number of pixels above or below the thresholds, because the undistorted image is the image that is viewed, analyzed, and stored. However, prior-art imaging systems typically count pixels of the distorted image, and therefore do not provide accurate pixel counts of the corresponding undistorted image. It is possible to count the pixels in the undistorted image, but this causes delays in the result and can thus delay the automatic exposure control loop. U.S. Publication No. 20140092226 by Kuriyama describes an automatic exposure control device that adjusts exposure values linearly based on a pixels distance from the center of an automatic exposure area. U.S. Publication No. 20140081083 by Morita et al. describes an exposure control process that can calculate exposure to control brightness corresponding to a particular area of a scope's image.
What is needed are improved ways to control exposure in devices having wide angle lenses.